1. Field of the Invention
This invention relates to a method and a device for the volumetric measurement of a fluid stream in a conduit, using a flowmeter of the type having a follower element driven by the fluid stream and performing an orbital movement at a speed proportional to the volume flow rate of the fluid stream.
2. Prior Art
An example of a flowmeter of this type is shown in U.S. Pat. No. 3,861,210. This flowmeter has a generally circular housing with a tangential inlet and an axial outlet. Within the housing there is a ball-shaped follower element which is driven by the fluid stream along a circular orbital path concentric with the outlet. During each completed orbital cycle the ball passes at least one sensing point having, for example, photoelectric or inductive sensors producing signals in response to the ball's passing the sensing point, which signals are detected and processed. The frequency at which the ball passes the sensing point, and thus the frequency or repetition rate of the signals, is proportional to the volume flow rate of the stream, and knowing the fluid volume that flows through the housing for each signal--this volume is hereinafter termed "incremental volume"--one can thus determine the volume flow rate from the repetition rate of the signals. Accordingly, the total fluid volume that flows through the housing during a measuring period can be determined either by multiplying the number of signals detected during the measuring period by the incremental volume or by multiplying the volume flow rate by the length of the measuring period.
A condition that has to be met in order that a correct value of the total volume may be obtained with varying values of the volume flow rate is that there is a direct and linear proportionality of the volume flow rate to the repetition rate at least over the range within which the volume flow rate varies or, in other words, that the incremental volume is independent of variations of the volume flow rate. In actual practice, however, this condition is not met, and particularly with low volume flow rates the deviation from the direct and linear proportionality may be substantial. In cases where great accuracy is required, a correction has therefore to be made.
In accordance with a known method for such correction of volume determination made by multiplying the volume flow rate by the repetition rate, a constant correction is added to the measured value of the repetition rate and the volume determination is then effected using the total value of the repetition rate thus obtained (U.S. Pat. No. 3,978,727). This method of correction does not always produce a sufficiently accurate correction, however, since it does not take into account the nonlinearity of the function representing the repetition rate versus the volume flow rate.